1. Technical Field
The present invention relates to semiconductor wafer processing equipment. More particularly, the present invention relates to a power loss recovery mechanism for a semiconductor wafer heater.
2. Description of the Prior Art
A heater may be used during semiconductor wafer processing to establish and maintain the wafer at a desired elevated temperature and thereby assure that wafer temperature is optimized to the particular process step being performed. FIG. 1 is a partially sectioned side elevation view of a prior art process environment 10 that includes a semiconductor wafer heating assembly.
During processing, the wafer 28 is rigidly supported by a pedestal, such as a heater assembly 22. The heater assembly comprises a large thermal mass that is maintained at a desired temperature such that the wafer, when placed on the heater assembly, is also maintained at the desired temperature during processing. Ideally, the heater assembly maintains a consistent temperature across the wafer surface, even though processing conditions might otherwise produce a thermal gradient or variation across the wafer surface. To this end, a conventional wafer heater assembly provides several temperature mediation mechanisms in addition to thermal mass, including:
1) a heat source 23, that may be an electrical heater or a conduit for circulating a heated fluid; PA1 2) a wafer backside thermal transfer fluid, such as a coolant or heating gas that is circulated by a conduit 25; and/or PA1 3) a vacuum 26 that draws the wafer into intimate contact with the wafer heater assembly.
Heater assembly temperature is monitored by a thermocouple 24. The thermocouple is placed in the heating assembly, preferably such that temperature at the heater assembly surface, i.e. at the wafer backside, is monitored.
The typical process chamber, such as that shown in FIG. 1, has a dome-shaped upper portion 33. A susceptor 34 that is positioned above the wafer is instrumental during processing in establishing a plasma within the chamber, for example during reactive ion etching. In the figure, a semiconductor wafer 28 is shown resting upon a plurality of support fingers 20, for example just prior to or after a processing step.
The wafer 28 is brought into processing position as a result of the compound movement of the support fingers 20 and a wafer heater assembly 22. This compound movement is effected by a drive 11 that is coupled to a drive shaft 15 by two pulleys 12, 13, and a belt 14. The drive 11 is operable to reversibly drive the shaft 15, which is part of a lead screw assembly 16. As shown by the arrows that are identified by numeric designators 35 and 36, the lead screw assembly 16 raises (and also lowers) the heater assembly, while a spring biased carriage assembly 17, 18, 19 raises (and also lowers) the lift fingers 20. This motion is described in more detail below. A bellows 30 surrounds and seals the heater assembly 22 from the ambient to help maintain a vacuum in the processing environment, and thereby prevent the entry of contaminating particles into the processing environment, while allowing ready movement of the heater assembly and lift fingers during wafer processing.
Operation of the heater assembly is as follows: after the wafer 28 has been placed on the support fingers 20, the shaft 15 is rotated by the drive 11 and belt 14 and pulley 12, 13 mechanism to move the lead screw assembly 16 upwardly, lifting a carrier 82, and correspondingly moving the heater assembly base 21 upwardly to raise the heater assembly 22 into a processing position directly beneath and supporting the wafer. At the same time, upward movement of the carrier 82 pushes the carriage 17, which pushes a push rod 18 and a platform 19 and, correspondingly moves the support fingers 20 upward, thereby raising the wafer upwardly. This compound motion continues until the carriage 17 hits a stop 83, at which point the carriage no longer moves upwardly and, therefore, the fingers 20 no longer move upwardly.
However, the heater assembly 22 continues to be raised into a position beneath the wafer. As the heater assembly moves upwardly, the upper surface of the heater assembly engages with the fingers 20 and lifts the fingers upwardly. This action causes the fingers to seat in a recess in the heater assembly surface, such that the wafer 28 rests on the heater assembly surface. Thus, when the wafer is in processing position, the support fingers have been lowered to the point that they are recessed into the heater assembly, and the wafer is resting within a well 27 formed in the upper surface of the heater assembly.
When wafer processing is completed, the above compound motion is repeated in reverse. That is, the heater assembly, is lowered such that the lift fingers emerge from the heater assembly and support the wafer. As the heater assembly continues to be lowered, the carriage assembly 17, 18, 19 is engaged and forced downwardly, such that the lift fingers are also lowered, and accordingly, the wafer is also lowered.
The above mentioned compound motion may also include a coordinated wafer clamping action as follows: After the wafer is positioned on the support fingers and prior to wafer processing, an actuator 31 that is linked to a wafer clamp assembly 32 is retracted, while at the same time the support fingers are lowered and the wafer heater assembly is raised. Retracting the actuator lowers the wafer clamp assembly into abutment with the edges the wafer, rigidly securing the wafer to the upper surface of the wafer heater assembly during processing.
Actual movement of the wafer clamping assembly may either be coordinated with that of the heater assembly and the support fingers, such that a three-way compound motion is achieved; or wafer clamping may take place after the wafer is placed on the surface of the heater assembly. Wafer clamping may also be accomplished by gravity, such that an actuator may not be required.
After wafer processing is completed, the steps above are reversed. That is, the wafer clamping assembly is lifted from the wafer surface; the wafer heater is lowered, while at the same time the support fingers are raised to lift the wafer from the heater assembly surface. Thereafter, the wafer is removed from the process chamber and another wafer may be loaded into the chamber for processing.
It is not uncommon in a typical wafer fabrication facility that electrical power to the facility will from time-to-time be interrupted, for example due to a power outage or as a result of tripping a circuit breaker or a process interruption. In the absence of electrical power the wafer heater assembly is no longer heated and it therefore begins to cool down. Processing temperatures are on the order of 475.degree. C. and wafer heater expansion is a commonly accepted phenomenon that is considered when designing and dimensioning the various components of the wafer heater assembly.
As discussed above, the wafer heater 22 surface defines a well 27 that is shaped to receive and align the wafer during processing. During an unintentional cool down of the wafer heater, for example during an interval of electrical power loss, the wafer heater assembly will begin to contract. As a result, the diameter of the wafer heater well becomes smaller, until the heater well diameter is less than that of the wafer. As the wafer heater assembly contracts, the inner surfaces of the well (i.e. 6 to 8 guide pins made of a hard material, such as Al.sub.2 O.sub.3) press against the edges of the wafer, and will eventually break the wafer if electrical power is not quickly restored. The consequences of such wafer breakage are not confined to the loss of the wafer itself. Wafer breakage in situ also necessitates shutting down the process environment for a thorough cleaning to remove any particles that may been produced during the wafer breakage because the particles pose a substantial threat of wafer contamination. This procedure is time consuming and therefore requires that the process chamber be out of service for an extended period of time 12 hours. As a result of such downtime, wafer throughput is impacted, degrading productivity and reducing profitability.
Additionally, power interruption also damages the heater assembly itself because the heater, which is made of aluminum, will give at high temperatures. Fragments of a shattered wafer are very sharp and will cut into the surface of the heater assembly, thereby ruining the flat, precision heater assembly surface that is necessary for the heater assembly to operate as a vacuum chuck.
A recovery system that would remove a wafer from the wafer heater well during a power outage or other service interruption before the wafer heater begins to contract and trap or break the wafer would be welcomed by the semiconductor manufacturing industry.